This paper proposes a relay-based scheme where a fixed number of relays are utilized to design an OFDM cooperative protocol. By exploiting limited feedback from the destination terminal (central node) such that each relay is able to help forward information of multiple sources in one OFDM symbol, the proposed cooperative protocol not only achieves full diversity but also efficiently utilizes available bandwidth. A practical relay assignment scheme is also proposed to specify the pairing of sources and relays in cooperative networks. The outage probability analysis of the proposed cooperative protocol is provided, and the analytical results are validated via simulations. Moreover, a closed-form lower bound on the outage probability of any relay-assignment schemes is established to provide a performance benchmark of the proposed cooperative protocol. Based on the outage probability analysis, the optimum relay location for the proposed relay-assignment scheme is determined. Both theoretical and simulation results show that in WLAN scenario the proposed cooperative protocol can achieve significant coverage extension compared to the direct transmission.

This paper provides performance analysis of multiband OFDM UWB system that takes into consideration the effect of intersymbol interference and imperfect frequency and timing synchronization. The system performance is analyzed under UWB multipath fading channels, as specified in the IEEE 802.15.3a channel standard. Average signal-to-noise ratio is first derived for the system. Then an approximation technique is employed to obtain a closed-form average bit error probability that provides a profound understanding of the performance of the multiband OFDM UWB system. Simulation results validate the theoretical analysis.

Extending lifetime of battery-operated devices is a key design issue that allows uninterrupted information exchange among distributed nodes in wireless networks. Cooperative communications has recently emerged as a new communication paradigm that enables and leverages effective resource sharing among cooperative nodes. In this paper, a general framework for lifetime extension of battery-operated devices by exploiting cooperative diversity is proposed. The framework efficiently takes advantage of different locations and energy levels among distributed nodes. First, a lifetime maximization problem via cooperative nodes is considered and performance analysis for M-ary PSK modulation is provided. With an objective to maximize the minimum device lifetime under a constraint on bit-error-rate performance, the optimization problem determines which nodes should cooperate and how much power should be allocated for cooperation. Since the formulated problem is NP hard, a closed-form solution for a two-node network is derived to obtain some insights. Based on the two-node solution, a fast suboptimal algorithm is developed for multi-node scenarios. Moreover, the device lifetime is further improved by a deployment of cooperative relays in order to help forward information of the distributed nodes in the network. Optimum location and power allocation for each cooperative relay are determined with an aim to maximize the minimum device lifetime. A suboptimal algorithm is developed to solve the problem with multiple cooperative relays and cooperative nodes. Simulation results show that the minimum device lifetime of the network with cooperative nodes improves 2 times longer than the lifetime of the non-cooperative network. In addition, deploying a cooperative relay in a proper location leads up to 12 times longer lifetime than that of the non-cooperative network.

Emerging ultra-wideband (UWB) technology offers a great potential for the design of high-speed short-range communications. However, in order for a UWB device to coexist with other devices, the transmitted power level of UWB is strictly limited by the FCC spectral mask. Such limitation poses a significant design challenge to any UWB system. An efficient management of the limited power is thus a key feature to fully exploit the advantages of UWB. In this paper, a cross layer multiuser multiband UWB scheme is proposed to obtain the optimal subband and power allocation strategy. Optimization criteria involve minimization of power consumption under the constraints on the packet error rate, the data rate, and the FCC limit. To ensure the system feasibility in variable channel conditions, an algorithm to jointly manage the rate assignment of UWB devices, subband allocation, and power control is proposed. A computationally inexpensive suboptimal approach is also developed to reduce the complexity of the problem, which is found to be NP hard. Simulation results under UWB channel model specified in the IEEE 802.15.3a standard show that the proposed algorithm achieves comparable performances to those of the complex optimal full search approach, and it can save up to 61% of transmit power compared to the current multiband scheme in the standard proposal. Moreover, the proposed algorithm can obtain the feasible solutions adaptively when the initial system is not feasible for the rate requirements of the users.

In this paper, we propose a relay-based scheme where a fixed number of relays are utilized to design a OFDM cooperative protocol. By exploiting limited feedback from the destination such that each relay is able to help forward information of multiple sources in one OFDM symbol, the proposed cooperative protocol not only achieves full diversity but also efficiently utilizes available bandwidth. A practical relay assignment scheme is also proposed to specify the pairing of sources and relays in cooperative networks. The outage probability analysis of the proposed cooperative protocol is provided, and the analytical results are validated via computer experiments. Moreover, a closed-form lower bound on the outage probability of any relay-assignment schemes is established to provide a performance benchmark of the proposed cooperative protocol. Based on the outage probability analysis, the optimum relay location for the proposed relay-assignment scheme is determined.

In this paper, cooperative sensor protocol and relay deployment in wireless sensor networks (WSNs) is proposed to improve the network lifetime. First, cooperative transmission is employed among sensor nodes. With an objective of maximizing the network lifetime under a constraint on bit-error-rate performance, we determine which sensors should cooperate and how much power to allocate for cooperation. A closed form solution is provided for a two-sensor WSN. Based on the obtained two-sensor solution, a fast suboptimal algorithm is developed for multiple sensor case. Moreover, the network lifetime is further improved by deploying cooperative relays in the network to help sensors transmit their information. The network lifetime is maximized by choosing the optimum location for each relay and optimally allocating the power that the relay helps each sensor. A suboptimal algorithm is proposed for the WSN with multiple relays. Simulation results show that the network lifetime of the proposed WSN with cooperative sensor employment improves 2.3 times compared with the non-cooperative WSN. In addition, deploying a cooperative relay in the proper location improves network lifetime up to 3 times of that for the non-cooperative WSN.

This paper proposes a threshold-based differential decode-and-forward cooperative scheme that efficiently exploits the cooperative relay channels via the use of a pre-determined decision threshold. In the proposed scheme, the source information is forwarded by the relay only if it is correctly decoded. The properly-designed threshold enables the destination to decide whether the received signal from the relay contains information such that the received signals from the source and the relay can be efficiently combined and jointly decoded. The bit error rate (BER) analysis of the proposed scheme is analyzed in case of differential M-ary phase shift keying signals. A tight BER approximation is established, and BER upper bound and lower bound are determined. Based on the tight BER approximation, joint optimum decision threshold and power allocation is numerically evaluated. Both analytical and simulation results reveal that the decision threshold and the power allocation depend on channel link qualities. Interestingly, when the link quality between the relay and the destination is very good, the effect of the threshold dominates the effect of the power allocation at high signal-to-noise ratio. Extensive simulation results are provided to validate the merit of the proposed scheme and confirm the theoretical analysis

In this paper, we study the impact of cooperative routing for maximizing the network lifetime in sensor network applications. We assume nodes in the network are equipped with a single omnidirectional antenna and they perform cooperative transmission to achieve transmit diversity. We propose a joint cooperative transmission and energy aware routing algorithm to prolong the network lifetime. In contrast to the previous works, our approach uses the maximum lifetime power allocation, instead of minimum power allocation. Using the maximum lifetime power allocation, the cooperative nodes allocate their transmit power according to the channel condition and the residual energy in the nodes. Our proposed scheme combines the maximum lifetime power allocation and the energy aware routing to maximize the network lifetime. We study the performance of the cooperative routing in terms of network lifetime (defined as the time until the first node dies) and the total delivered packets before the first node dies. We demonstrate that the proposed solution achieves 1 ~ 3.5 and 1 ~ 2 times longer network lifetime and more total delivered packets compared to noncooperative routing, when it is used with MTE and FA algorithms, respectively. Furthermore, the maximum lifetime power allocation achieves 1 ~ 2 times longer lifetime, compared to minimum power allocation in MTE and FA routing schemes. We also provide distributed implementation of the proposed algorithm.

This paper proposes a threshold-selection relaying scheme, in which each relay decides whether to forward the source information by comparing the received signal power with a decision threshold. The challenging problem is to design the threshold such that the relay is able to forward only correctly decoded information. In this work, bit-error-rate (BER) performance analysis is provided for the proposed scheme with BPSK signals. We develop a BER upper bound which is tight over the entire range of signal-to-noise ratio values. Based on the established BER formulation, we jointly determine optimum decision threshold and power allocation. An interesting result is that the effect of optimum threshold dominates that of optimum power allocation, especially when the relay is close to the destination. For example, in case of equal power allocation, the proposed scheme with optimum threshold yields 10 dB performance improvement over the fixed relaying scheme without Q threshold at a BER of 10-2. If both the power allocation and the threshold are jointly optimized, then the performance of the proposed scheme can be further improved by 2 dB.

Due to limitation on transmitted power level, any UWB system faces major design challenges in achieving wide coverage while assuring an adequate system performance. In this paper, an employment of cooperative communications in UWB is proposed to enhance the performance and the coverage of UWB by exploiting the broadcasting nature of wireless channels and the cooperation among UWB devices. Symbol-error-rate (SER) performance analysis and optimum power allocation are provided for cooperative UWB multiband OFDM systems with decode-and-forward cooperative protocol. To capture the multipath-clustering phenomenon of UWB channels, the SER performance is characterized in terms of cluster and ray arrival rates. An optimum power allocation is determined based on two different objectives, namely minimizing the overall transmitted power and maximizing the system coverage. Furthermore, an improved cooperative UWB multiband OFDM scheme is proposed to take advantage of unoccupied subbands. Simulation results are shown to validate the theoretical analysis.

This paper provides a novel performance analysis for UWB systems that successfully captures the unique multipath-rich property and multipath-clustering phenomenon of UWB channels. Using the Saleh-Valenzuela model, we characterize the pairwise error probability and the outage probability for UWB systems employing multiband OFDM based on the cluster arrival rate, the ray arrival rate within a cluster, and the cluster and ray decay factors. Furthermore, the effect of random-clustering phenomenon on the performance of UWB-MIMO systems is analyzed. The theoretical results reveal that regardless of the clustering behavior of UWB channels, the diversity gain can be improved by increasing the number of jointly encoded subcarriers, the number of jointly encoded OFDM symbols, or the number of antennas. The coding gain on the other hand, depends heavily on the cluster-arriving channels. Extensive simulation results are provided to support the theoretical analysis.

In this paper, we propose an OFDM cooperative protocol that not only achieves full diversity but also efficiently utilizes available bandwidth. The proposed protocol exploits limited feedback from the destination terminal (central node) such that each relay is able to help forward information of multiple sources in one OFDM symbol. To specify how relay-source pairs should be assigned, we propose two practical relay-assignment schemes, including fixed-location scheme in which the relays are fixed at optimum locations, and centralized-control scheme in which the relays are assigned by the central node. We provide outage probability analysis of the proposed protocol in wireless indoor environment. Moreover, a lower bound on the outage probability of any relay-assignment schemes is established, and the performance of the proposed relay-assignment schemes is analyzed. We also investigate the application of the proposed protocol to enhance the performance of UWB systems. In UWB wireless indoor scenarios, both theoretical and simulation results show that the proposed cooperative protocol can achieve 75% power saving and 200% coverage extension compared to the noncooperative UWB system proposed in the IEEE 802.15.3a standard.

This paper provides a novel performance analysis for ultra-wideband (UWB) systems that successfully captures the unique multipath-rich property and multipath-clustering phenomenon of UWB channels. Using the Saleh-Valenzuela model, we characterize pairwise error probability and outage probability for UWB systems employing multiband OFDM based on the cluster arrival rate, the ray arrival rate within a cluster, and the cluster and ray decay factors. Furthermore, an approximation technique is established, which allows us to obtain closed-form performance formulations that provide insightful understanding of the effect of channel characteristics on the performances of UWB systems. Finally, we characterize the effect of random-clustering phenomenon on the performance of UWB-multiple-input-multiple-output systems. The theoretical results reveal that regardless of the clustering behavior of UWB channels, the diversity gain can be improved by increasing the number of jointly encoded subcarriers, the number of jointly encoded orthogonal frequency-division multiplexing symbols, or the number of antennas. The coding gain on the other hand, depends heavily on the cluster-arriving channels. Extensive simulation results are provided to support the theoretical analysis.

Cooperative transmission has been shown to be able to greatly improve the system performance by exploring the broadcasting nature of wireless channels and cooperation among users. While most existing works concentrate on improving the peer-to-peer link quality, we focus, in this paper, on resource allocation among users such that the system performance can be improved. In this work, two important questions are answered: who should help whom among the distributively located users; and how the users should cooperate to improve the performance. To quantify the questions, a power management problem is formulated over a multiuser OFDM network to minimize the overall system transmit power under the constraint of each user's desired transmission rate. Then, we develop an algorithm to find solutions for a two user case. From the simulation results, the proposed scheme achieves up to 50% overall power saving for the two-user system.

The emerging ultra-wideband (UWB) system offers a great potential for the design of high-speed short-range communications. However, UWB faces a significant challenge in achieving a low transmit power level, while assuring an adequate system performance. An efficient management of the limited power is thus a key feature to fully exploit the advantages of UWB. In this paper, a cross layer multiuser multiband UWB scheme is proposed to obtain the optimal subband and power allocation strategy. Optimization criteria involve minimization of power consumption under the constraints on the packet error rate, transmission rate, and FCC regulations. To reduce the problem complexity, which is found to be NP hard a computationally inexpensive suboptimal approach is developed. Simulation results under UWB channel model specified in the IEEE 802.15.3a standard show that the proposed algorithm achieves comparable performance to that of the complex optimal full search approach, and it can save up to 61% of transmit power compared to the multiband scheme in the standard proposal.

In this paper, we propose a general framework to analyze the performance of multiband UWB-MIMO systems regardless of specific coding schemes. A combination of space-time-frequency (STF) coding and hopping multiband OFDM modulation is also proposed to fully exploit all of the available spatial and frequency diversities, richly inherent in UWB environments. We quantify the performance merits of multiband UWB-MIMO systems in case of Nakagami-m frequency-selective fading channels. We show that the maximum achievable diversity of the proposed system is the product of the number of transmit and receive antennas, the number of multipath components, and the number of jointly encoded OFDM blocks. Interestingly, theoretical result shows that the diversity gain does not severely depend on the fading parameter m.. Finally, simulation results are presented to support the theoretical analysis.

In this paper, we analyze the performance of ultra-wideband (UWB) multiple-input multiple-output (MIMO) systems employing various modulation and multiple access schemes, including time hopping (TH) binary pulse position modulation (BPPM), TH binary phase shift keying (BPSK), and direct sequence (DS) BPSK. We quantify the performance merits of UWB space-time (ST) systems regardless of the specific coding technique. We introduce a framework that enables us to compare UWB-MIMO systems with conventional UWB single-antenna systems in terms of diversity and coding gains. We show that the combination of ST coding and Rake architecture is capable of exploiting spatial diversity as well as multipath diversity, richly inherent in UWB environments. We find the upper bound on the average pairwise error probability (PEP) under the hypothesis of quasi-static Nakagami-m frequency selective fading channels. Finally, simulation results are presented to support the theoretical analysis.

In this paper, we analyze the performance of ultra-wideband (UWB) multiple-input multiple-output (MIMO) systems employing various modulation and multiple access schemes, including time hopping (TH) M-ary pulse position modulation (MPPM), TH binary phase shift keying (BPSK), and direct sequence (DS) BPSK. We quantify the performance merits of UWB space-time (ST) systems regardless of the specific coding scheme. For each modulation scheme, we introduce a framework that enables us to compare UWB MIMO systems with conventional UWB single-antenna systems in terms of diversity and coding gain. Moreover, we adopt the real orthogonal design (ROD) as the engine code for UWB ST codes. We find the closed form expressions of the average pairwise error probability (PEP) bound for all of the schemes under the hypothesis of Nakagami flat fading channels. The extension to frequency-selective fading case is also briefly addressed. Finally, simulation results are presented to support the theoretical analysis.